docs: def handle_annotation_key(self, line: str, position: int, tokens: ParseResults) -> ParseResults:
          """Handle an annotation key before parsing to validate that it's either enumerated or as a regex.
  
          :raise: MissingCitationException or UndefinedAnnotationWarning
          """
          key = tokens['key']
          self.raise_for_missing_citation(line, position)
          self.raise_for_undefined_annotation(line, position, key)
          return tokens